Towards a modular theory of trophic interactions

Abstract

1. Species traits and environmental conditions determine the occurrence and strength of trophic interactions. If we understand the relationship between these factors and trophic interactions, we can make more accurate predictions and build better trophic-interaction models.2. We can compare traits and conditions by considering their effect on different parts (steps) of a trophic interaction, such as the steps search and pursuit. By linking traits to relevant steps, we can use these relationships to build trophicinteraction models. Currently, this is done ad hoc, defining steps based on the species and traits of interest. This makes it difficult to compare across traits and species and gain an overarching understanding of how traits and the environment drive trophic interactions.3. We present a comprehensive approach for the explicit choice of interaction steps and species traits or environmental conditions, which is readily integrated into existing models. The core of this framework is that it is modular; we present eight steps that occur in all trophic interactions and use them to build a modular, general dynamic model. When applying the framework, one explicitly selects only the most relevant steps and uses those to build a specific model. 4. To build our modular framework, we revisit and expand the functional and numerical response functions, dividing the trophic interaction into eight steps: (1) search, (2) prey detection, (3) attack decision, (4) pursuit, (5) subjugation, (6) ingestion, (7) digestion and (8) nutrient allocation. Together these steps form a general dynamical model where trophic interactions can be explicitly parameterized for multiple traits and environmental factors. We then concretize this approach by outlining how a specific community can be modelled by selecting key modules (steps) and parameterizing them for relevant factors. This we exemplify for a community of terrestrial arthropods using empirical data on body size and temperature responses.5. With species interactions at the core of community dynamics, our modular approach allows for quantification and comparisons of the importance of different steps, traits, and abiotic factors across ecosystems and trophic-interaction types, and provides a powerful tool for trait-based prediction of food-web structure and dynamics.

Beyond body size—new traits for new heights in trait-based modelling of predator-prey dynamics

“…[ 59 ]), then a model including microhabitat use will have vastly different predictions from one not including microhabitat use. As we begin to build our library of trait-based models and our understanding of where and how different traits affect different parts of the predation process, we will increase our capacity to predict and manage the dynamics of ecological communities in diverse and changing ecosystems [ 8 ].…”

“…Adding the right traits may even remove the need for variable R opt values, taking us closer to a fully trait-based model. Of course, adding extra traits comes with extra work associated with measuring them, and the optimal traits for a given study may vary (see [ 8 ] for a framework for selecting traits).…”

“…Bembidion has higher final experimental population sizes than predicted by most models, suggesting Bembidion likely has traits, such as a tough cuticle [ 18 ], which decrease mortality rate from other predators but are not included in the model. These traits—foraging mode and defensive traits—are therefore likely candidates to be integrated into further iterations of trait-based models [ 8 ]. Adding the right traits may even remove the need for variable R opt values, taking us closer to a fully trait-based model.…”

“…But, since we can i) make predictions using the models and ii) explicitly test those predictions, the ultimate test of the validity of our models should be based on further, iterating cycling between predictions and further experiments. A model built on traits also allows us to predict how interactions and dynamics will respond to a change in traits or the environment [ 8 ]. For example, if predator and prey change their microhabitat use to overlap more or less (as may be caused, for example, by differences in landscape [ 35 ] or loss of habitat e.g.…”

“…Although dynamic food-webs can accurately describe observed interactions, their complexity has made it unwieldy to map the abundance dynamics of diverse predator-prey assemblages in nature. Recent developments in food-web ecology are now offering a potential cure for this ‘plague of parameters’ [ 6 ] through trait-based approaches [ 7 , 8 ]. Although any traits can be used in this approach, allometric (body-size based) approaches are the most common of these approaches and have been used on a wide range of species and communities [ 3 , 9 – 11 ].…”